1. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly to the structure of a semiconductor chip with a heterojunction bipolar transistor (HBT) mounted on a semi-insulating compound semiconductor substrate.
2. Description of the Prior Art
In recent years, there has been a demand for high-output-power semiconductor devices that can operate in a GHz band in view of the widespread use of consumer products that operate in an ultra-high frequency band, such as portable telephone units.
Heterojunction bipolar transistors (HBT) have an emitter-to-base junction comprising a different material such as AlGaAs or GaInP/GaAs for high electron mobility and a greater emitter band gap than a base band gap for high electron injection efficiency. Therefore, since the heterojunction bipolar transistors can have a high output power rate ranging from 1 W to 2 W in the GHz band, they are attracting attention as meeting the above demand.
Various documents, e.g., IEEE ELECTRON DEVICE LETTERS, Vol. 14, No. 10, October 1990, pages 493.sup..about. 495, and Japanese patent No. 2522280, disclose heterojunction bipolar transistors using compound semiconductor materials. The disclosed heterojunction bipolar transistors comprise an n.sup.+ -GaAs subcollector layer, an n-GaAs collector layer, a p.sup.+ -GaAs base layer, an n-AlGaAs emitter layer, an n.sup.+ -GaAs emitter contact layer, etc. that are successively deposited in a mesa structure on a semi-insulating GaAs substrate.
The heterojunction bipolar transistors suffer the problem of heat radiation because their current density is very high. To solve this problem, the heterojunction bipolar transistors disclosed in the above documents have a heat-radiating electrode mounted on the semi-insulating GaAs substrate. A cell region where the collector layer, the base layer, and the emitter layer are positioned in the mesa configuration, a base wiring electrode, and a collector wiring electrode are all covered with an insulating film. An opening is defined in the insulating film in communication with the emitter layer and the heat-radiating electrode, and the insulating film is covered with a thick gold-plated layer. Heat generated from the emitter layer in the cell region is radiated from the thick gold-plated layer, and is also transferred through the heat-radiating electrode to the semiconductor chip and a package in which the semiconductor chip is fixedly disposed, thereby cooling the region where the heat is generated.
The heterojunction bipolar transistors disclosed in the above documents have a plurality of stripe-shaped cell regions on a semiconductor chip, each comprising a mesa structure of collector, base, and emitter layers, collector wiring electrodes connected to the collector layers, base wiring electrodes connected to the base layers that serve as cells. Heat-radiating electrode regions are disposed in outer peripheral areas of the semiconductor chip on which the stripe-shaped cell regions, the collector wiring electrodes, and the base wiring electrodes are disposed. The stripe-shaped cell regions, the collector wiring electrodes, and the base wiring electrodes are covered with an insulating film, which is covered with a thick gold-plated layer over its entire surface.
Specifically, the emitter layers of the stripe-shaped cell regions and heat-radiating electrodes on both ends of the semiconductor chip are connected by the thick gold-plated layer which is shaped like a bridge and has a thickness of about 22 .mu.m. Heat that is generated in the vicinity of base-to-collector junctions is radiated into the air from the thick gold-plated layer that is deposited on the entire surface of the semiconductor chip, and is also transferred from the heat-radiating electrodes on the both ends of the semiconductor chip to a package on which the semiconductor chip is fixedly mounted.
While the heterojunction bipolar transistors can produce high output power in the ultra-high frequency band, they are subject to positive feedback of the output power due to a temperature rise. Generally, high-output-power transistors comprise a number of unit transistor cells each having emitter and base regions disposed in a collector region. These unit transistor cells are connected into a single transistor. When the temperature of one of the unit transistor cells rises, a collector current with respect to the same base bias voltage increases, resulting in a temperature rise. The temperature rise then causes the collector current to increase. This positive feedback phenomenon causes the current to concentrate on the single unit transistor cell until the high-output-power transistor will suffer a thermal breakdown.
To solve the above problem, it is important that the radiation of heat from the unit transistor cells that are dispersed widely on the transistor chip be made as uniform as possible for thereby eliminating localized temperature rises which would otherwise be developed if the heat radiation were not uniform on the transistor chip.